The general problem solved by this invention is that of out-of-band intermodulation (IM) noise generated by radio transmitters due to the effects of circuit non-linearities in the power amplifier. All transmitter power amplifiers exhibit intermodulation noise to some degree. Digital Communications by Satellite, by J. J. Spilker, Prentice Hall, Inc., Englewood Cliffs, N.J., 1977, pages 220-264 provides an overview of the theoretical aspects of intermodulation noise generation in non-linear amplifiers. FIG. 1 from this reference and labelled "Prior Art", shows the general nature of the output spectrum of the theoretical, non-linear amplifier with a wideband input signal. The "ears" extending to either side of the output spectrum are the out-of-band component of intermodulation noise under discussion. This noise can interfere with users in adjacent bands of the radio spectrum. Power amplifiers (PA's) must therefore be designed to meet specifications on the amount of out-of-band intermodulation noise they can emit.
In the particular application being addressed here, the radio transmitter is in a satellite communications downlink. The transmitting band is adjacent to a band that has been set aside for radio astronomy (RA) observations. Any man-made interference in the radio astronomy band must be kept to exceedingly low levels. While in the embodiment disclosed herein, the invention minimizes power amplifier generated intermodulation noise in the radio astronomy band, the invention can be applied to minimizing the out-of-band intermodulation noise of radio transmitters in general.
The non-linear circuit effects in transmitter power amplifiers can be classified as saturation, AM-AM conversion and AM-PM conversion. FIG. 2 is from the above reference and labeled "Prior Art", and is a theoretical model of a power amplifier. It illustrates the three classes of non-linear effects. Saturation is included in FIG. 2 with the "Amplitude Non-linearity, F(z)".
The object of the present invention is to provide a method and apparatus for suppression of intermodulation noise in radio frequency power amplifiers.
The present invention addresses saturation, which is a very significant, if not the dominant, non-linear effect contributing to intermodulation noise in many transmitter circuits.
In this invention, the power amplifier input signal is assumed to be a wideband signal with, for example, a Raleigh-distributed amplitude envelope. The Raleigh distribution characterizes some important classes of waveforms such as multi-carrier frequency division multiplexed (FDM) signals and code division multiplexed (CDM) signals. A feature of a Raleigh distributed signal is that it has occasional envelope peaks that are much higher than the average amplitude. These peaks are clipped by the power amplifier saturation characteristic. This clipping is one of the major sources of intermodulation noise in the power amplifier.
The out-of-band intermodulation noise could be removed by a filter following the power amplifier, but the insertion loss of typical filters often makes this an unattractive solution because it reduces the overall efficiency of the transmitter to unacceptable low levels.
The clipping effect can also be minimized by designing the power amplifier saturation level to be much higher than the average envelope level, however this is usually an unacceptable solution because the power amplifier would then be operating at very low efficiency.
With practical power amplifiers, therefore one must do a trade-off which involves setting the average signal envelope level (or equivalently the power amplifier output signal power level) relative to the power amplifier saturation level in such a way that it maximizes efficiency on the one hand and minimizes intermodulation noise on the other.
The present invention allows one to operate the power amplifier at higher efficiency for a given intermodulation noise level or equivalently, to obtain lower intermodulation noise at a given efficiency. The technique is effective for Raleigh-distributed signals or for any other signal waveform that has a high peak-to-average envelope distribution.